As described in U.S. Pat. No. 3,800,586, pressure decay leak testing is a way of determining leakage in a test item by pressurizing the test item to a given pressure, closing off the pressure source, and monitoring the decay in pressure which is a function of leakage. However, there are some leak testing applications in which the part to be tested is not at ambient temperature or the temperature of the testing fluid, but has a temperature different from ambient temperature. If this non-ambient test item is pressurized with air at ambient temperature, the test item will tend to heat or cool the air causing pressure changes due to temperature differences thereby causing an error in the pressure decay measurement.